Effects Of Forging Temperature And Heat Treatment Process On Microstructure And Mechanical Properties Of GH4706 Superalloy

Heavy-duty gas turbine is the most efficient heat-power conversion power generation equipment at present.With the improvement of thermal efficiency and power of gas turbine,the selection material of large size turbine disc,as one of the key hot end components,has been gradually upgraded to GH4706 alloy.GH4706 alloy(the corresponding brand Inconel 706)is a precipitation-strengthened Fe-Ni base deformation superalloy,which can be used in the preparation of large size turbine disks.However,With the expansion of the size,how to adjust the hot process parameters to obtain good microstructure to improve mechanical property has been a blank in domestic research.In view of this,hot compression experiment,upsetting-drawing and heat treatment experiment are designed for GH4706 alloy in this paper to study the effect of hot deformation and heat treatment in actual production for supersized turbine disc forgings.Under different upsetting-drawing temperatures,solution treated process and stabilization treatment process,the change of microstructure and its effect on mechanical property such as tensile property and stress rupture property are analyzed.In order to provide data and principle support for the localization of oversize disk foegings.The results show that GH4706 alloy generates dynamic recrystallization obviously during the hot deformation process,the true stress-true strain curve shows the steady-state rheological feature.The constitutive relation can be expressed by the hyperbola sine equation in the Arrhenius equation.When the deformation temperature is 950～1150℃ and the strain rate is 0.001～1 s-1,the alloy deformation activation energy is 380.61 kJ/mol and stress index is 3.82.The constitutive equation is applied to the numerical simulation,which provides a theoretical basis for the optimization of hot processing technology of the supersized GH4706 alloy turbine disc forgings.After the simulation,the downspeed classifying system is actually adopted to solve the problems of equipment travel and load.The fabrication of the 10-ton superalloy disk forging is successfully completed for the first time in our country.the recrystallized volume fraction is the largest in the 1060℃ temperature forging,the next in the 1110℃ temperature,and the lowest in the 1010℃ temperature.After solution treated at 980℃ for 2 h,the recrystallization volume fraction in 1010℃ is higher than that in 1110℃ temperature,but it’s the largest in 1060℃ temperature,but the CSL under 1060℃ temperature forging condition is significantly reduced.Thedynamic recrystallization nucleation of GH4706 alloy in 1010℃ is mainly the discontinuous dynamic recrystallization(DDRX)mechanism,and the continuous dynamic recrystallization(CDRX)and DDRX jointly promote the dynamic recrystallization nucleation in 1110℃ temperature.New grains nucleation in 1060℃ is due to the change of twin boundary orientation into large angle grain boundary during hot deformation.With the increase of solution temperature,the room temperature tensile strength and stress rupture property decrease,and the room temperature impact absorbed decrease greatly at 940℃.The size of η phase gradually decrease with the increase of solution temperature.At 1110℃ temperature,the γ’ phase is precipitated at the twin boundary to straighten the grain boundary,the stress rupture property of the alloy reduces obviously,while the prolongation of solution time has a little effect on the microstructure and properties of GH4706 alloy.The stress rupture fracture morphology at 940℃ is cup cone shape,and the fracture morphology at other temperatures is classical fracture morphology,which consists of intergranular fracture zone,transgranular fracture zone and instantaneous fracture zone.After stabilizing treatment,the η phase is distributed as a single sheet in the grain boundary,and the y’ phase is refined,which can significantly increase the stress rupture life of the GH4706 alloy.By observing the fracture morphology,it can be found that although the fracture is still intergranular,the stepped quasi cleavage fracture has appeared.The improvement of the stress rupture life comes from the joint action of grain boundary strengthening and intracrystalline strengthening.